A Doherty amplifier is widely used as an amplifier that operates with high efficiency while maintaining the linearity. The Doherty amplifier has a structure in which a carrier amplifier and a peak amplifier are connected in parallel between an input terminal and an output terminal. Further, the Doherty amplifier has a transmission line for matching the output impedance between the carrier amplifier and the peak amplifier. In the Doherty amplifier, the carrier amplifier and the peak amplifier operate when an output is at a high level, and only the carrier amplifier operates and the peak amplifier stops operating when an output is at a low level. By such operation, the Doherty amplifier can operate with high efficiency even when the output power is significantly lower than the saturation power.
Patent Literature 1 discloses a technique related to a Doherty amplifier capable of high-efficient operation even when a signal to be amplified is a high frequency band signal. The Doherty amplifier disclosed in Patent Literature 1 has two paths that are connected in parallel between an input terminal and an output terminal. A carrier amplifier including a first transistor and a first transmission line having an electrical length that is about ¼ wavelength of the frequency of an input signal are connected in series to one path. A second transmission line having an electrical length that corresponds to a phase difference of the first transmission line and a peak amplifier including a second transistor are connected in series to the other path. The first transistor and the second transistor are placed so that the output direction vector of a signal to be transmitted is 180 degrees.